US researchers have created exotic new versions of atomic nuclei including one previously thought to not exist.

The three new isotopes of magnesium and aluminium suggest other heavy variants of everyday elements could be created.

The new nuclei existed for only a fraction of a second and were created by smashing atoms at half the speed of light in a particle accelerator.

The work, published in the journal Nature, may give astrophysicists insights into the workings of stars.

"There is a second way to create super heavy nuclei: explosions in stars," said Paul Henri Heenen, a nuclear physicist at the Universite Libre de Bruxelles.

The lab experiments mimic these violent events and therefore can give astrophysicists clues about stellar processes.

The experiments will also allow theoretical physicists to test the validity of their nuclear models.

"The limit of stability of matter may be further out than previously expected," wrote Professor Dave Morrissey, one of the authors of the paper.

"It shows how much mystery remains about atomic nuclei."

Chasing heavier atoms

Physicists have known since the beginning of the 20th Century that the atomic nucleus is formed from two basic particles - protons and neutrons - that are held together by a strong nuclear force.

It is also an established fact that every atom of the same chemical element has a unique number of protons, but can have different number of neutrons - when that happens the two atoms are called isotopes.

But from the 1930s, scientists studying nuclear forces have been trying to establish the limit of how many neutrons a given number of protons can bind, a basic property of matter known as the neutron-drip line.

Prof Dave Morrissey: Heavier isotopes may be out there

"The knowledge of these drip lines is crucial to our understanding of the force that binds a nucleus," said Professor Heenen

But even today, scientists are not sure exactly which combinations of protons and neutrons can make up most atomic nuclei.

One way of addressing this question is to find out how many neutrons can be artificially loaded on to nuclei of so-called quotidian elements, which contain the same number of protons but different numbers of neutrons.

Accelerating particles

This is done using a circular particle accelerator known as a cyclotron that bombards heavy atoms with other materials at very high speeds. This triggers nuclear reactions that break the initial atoms into pieces and create new heavy nuclei.

Using this technique, the Michigan State University team created and detected a super heavy isotope of magnesium, magnesium-40, containing 12 protons and 28 neutrons.

Magnesium usually contains 12 neutrons.

They also found heavier counterparts of aluminium - aluminium-42, with 13 protons and 29 neutrons and aluminium-43, with 13 protons and 30 neutrons. Aluminium normally contains 14 neutrons.

Current theories suggested aluminium-42 was unlikely to exist.

"At this point, we can't define the end of magnesium and aluminium isotopes," said Dr Morrissey.

"We need to upgrade the measurement and detection techniques for the next experiments which will allow us to detect even more heavier isotopes," he added.